Disturbance to the ⁴⁰Ar/³⁹Ar system in feldspars by electron and ion beam irradiation

Flude, S., Sherlock, S.C., Lee, M.R. and Kelly, S.P. (2013) Disturbance to the ⁴⁰Ar/³⁹Ar system in feldspars by electron and ion beam irradiation. Chemical Geology, 355, pp. 1-12. (doi: 10.1016/j.chemgeo.2013.07.003)

Abstract

The extent to which intracrystalline microtextures influence the diffusion of radiogenic Ar within alkali feldspars from slowly cooled igneous rocks is a long standing question in thermochronology. By combining high-resolution electron microscopy with in-situ UV-laser ablation microprobe ⁴⁰Ar/³⁹ analysis the interplay of microtextures with isotope ages can be studied directly, enabling some of the assumptions underlying ⁴⁰Ar/³⁹ thermochronological techniques to be tested and allowing deduction of thermochronological and geological histories. However, there are numerous potential mechanisms by which a sample can be damaged and its Ar-isotope system disturbed by such microscopy techniques. To test this hypothesis, perthitic alkali feldspars from the 270–280 Ma Dartmoor Granite, UK, and gem-quality orthoclase from Itrongay, Madagascar (~ 470 Ma) were polished with colloidal silica or etched with hydrofluoric acid and irradiated with electrons and Ga+ ions. The accelerating voltages and currents used were typical of those for electron beam imaging by scanning electron microscopy, X-ray analysis and mapping by electron probe, and for extraction of foils using the focused ion beam technique. No disturbance to the Ar-isotope system was observed for Ga+ ion irradiation, or for low-resolution SEM imaging, but electron irradiation of small areas for long durations, as occurs during extended high-magnification SEM imaging, was found to disturb the Ar-isotope system over hundreds of micrometre sized areas by the addition of K and atmospheric Ar, producing anomalously young apparent ⁴⁰Ar/³⁹ages. The best explanation for this age disturbance is electromigration of K and implantation of atmospheric Ar during sample charging

• Item Type: Articles
• Keywords: feldspar, electron beam
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Lee, Professor Martin
• Authors: Flude, S., Sherlock, S.C., Lee, M.R., and Kelly, S.P.
• Subjects: Q Science > QE Geology
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
• Research Group: Earth Systems
• Journal Name: Chemical Geology
• Publisher: Elsevier
• ISSN: 0009-2541
• ISSN (Online): 1872-6836